1. Field of the Invention
The present invention relates to comb-form spectrum communication systems using repeated complementary sequence modulation and relates to, in particular, means for solving the communication interruption caused by so-called near-far problem.
2. Description of the Background Art
In recent mobile communication systems such as a mobile telephone and a PHS (Personal Handyphone System), a Time Division Multiple Access (to bereferred to as ‘TDMA’ hereafter) system is adopted so as to provide a necessary channel capacity for the system. The TDMA system is designed in order to allow a plurality of users to share a predetermined, assigned frequency band, so that the time axis of the signal is divided to thereby assign the divisions to users, respectively. The usable frequency band is, however, limited and the number of time divisions is also limited technically, for which reason the number of channels which can be assigned to users is limited, as well.
In recent years, as the user population of mobile communication systems stated above increases, there has been proposed a Code Division Multiple Access (to be referred to as ‘CDMA’ hereafter) system so as to provide the necessary channel capacity for the system. A CDMA system is designed in order to allow a plurality of users to share the same band, so that the users are identified with address spteading codes (inherent codes) assigned to them, respectively. Therefore, to facilitate the identification of the inherent codes, the inherent codes are made on a clock frequency of higher than that of an information signal, for example, two or three MHz. The information signal is multiplied by the inherent codes to thereby increase the bandwidth of the transmission signal (or to spread the spectrum) and it is transmitted to a transmission path. Then, at a receiver, the correlation characteristics of the received signal is obtained using matched filters etc. and the inherent codes are thereby demodulated. As stated above, since a CDMA system allows a plurality of users to share the same band, the number of users per bandwidth can probably increase compared with a TDMA system. Nevertheless, the problem of CDMA systems is that the number of simultaneous communication channels cannot increase due to the interference that are the signals coming from other users which share the same band, and also due to the near-far problem which will be described later.
FIG. 10 is a functional block diagram showing an example of the constitution of a conventional CDMA system. In FIG. 10, four users are assumed. Since the following explanation will be given for a case where information signals are transmitted from a user A to a user B and from a user D to a user C, respectively, the receivers of users A and D and the transmitters of users B and C are not shown in FIG. 10.
In the CDMA system shown therein, each user possesses a transmitter 105 (205) serving as a transmitting system and consisting of the first multiplier 102 (202) which multiplies an transmission information signal a (b) outputted from a transmitting information generator 100 (200) by the output signal of the first spreading code (PN code) generator 101 (201) which generates inherent codes with a time width of Δt, assigned to respective users, and the second multiplier 104 (204) which multiplies the output signal of the first multiplier 102 (202) by the output carrier signal of the first local signal generator 103 (203).
Also, each user possesses a receiver 116 (216) serving as a receiving system and consisting of the third multiplier 112 (212) which multiplies a received signal 110b (110c) coming from a transmission path 110 which takes a space as the medium, by the output signal of the second local signal oscillator 111 (211), and a matched filter 115 (215) which is composed of an integrator 114 (214) connected to the fourth multiplier 113 (213) multiplying the output signal of the third multiplier 112 (212) by the output signal of the second spreading code (PN code) generators 117 (217) generating the inherent code.
The required conditions for the above-described spreading codes are: (1) there are a lot of combinations of codes so that inherent codes can be assigned to a lot of users; (2) cross-correlation is so little that the code of a user can be discriminated from that of another user; (3) auto-correlation to the same codes is impulsive so as to track the signal addressed to the desired station and to facilitate the demodulation; (4) a code is as random and long in length as possible to prevent the third party from eavesdropping the communications content, and so on. Generally, PN (pseudo-noise) codes are utilized as codes satisfying the above conditions.
Next, the operation of the CDMA system shown in this example will be described. First, consider that user A who transmits an information signal a to user B. At the transmitter 105 of user A, the code generated at the first PN code generator 101 is set to an inherent code Mb assigned to user B. The inherent code Mb is multiplied by the information signal a at the first multiplier 102 to thereby spread the spectrum, and the frequency of the resultant signal is transformed (modulated) to a transmission frequency by both the second multiplier 104 and the first local carrier signal generator 103, and then the resultant output is sent out to the transmission path 110.
When receiver 116 of user B receives the said transmission signal, a received signal 110b is outputted to matched filter 115 after the frequency is transformed (demodulated) by both oscillator 111 having the same output frequency as the modulation frequency f0, and multiplier 112. Matched filter 115 functions as a time correlator in terms of operational principle (for which detail, see, for example, “Communication System”, page 297, B. P. Lathi, translated in Japanese by Sonosuke YAMANAKA and Koichi USAMI, McGraw-Hill Kogakusha, October 1981) and PN code generator 117 outputs the inherent code Mb assigned to the user B's station. As a result, the output of the auto-correlation characteristics of the inherent code sequence Mb is produced from matched filter 115.
FIG. 11 shows an example of the auto-correlation characteristics of the PN code which shows little correlation with a sequence shifted in phase by more than one chip. Consequently, if the same code as the inherent code assigned to the user B's station is inputted to the receiver, the matched filter produces the output of sharp auto-correlation characteristics, whereby the receiver can easily determine whether the received signal is addressed to the user B's station or not.
Now consider that an information signal b is transmitted from user D to user C while the information signal a is transmitted from user A to user B as stated above, the code of PN code generator 201 is set to an inherent code Mc assigned to user C at transmitter 205 of user D as in the case of the transmission operation of transmitter 105 of user A. The inherent code Mc is multiplied by the information signal b at multiplier 202 to thereby spread the spectrum and, at the same time, the frequency of the resultant signal is transformed (modulated) to a transmission frequency by both multiplier 204 and the output signal of local signal generator 203. Then the resultant transmission signal is transmitted to transmission path 110.
Accordingly, if receiver 216 of user C receives the signal transmitted from user D, PN code generator 212 outputs the inherent code Mc assigned to the user C's station as a spreading code. Thus, by performing the same operation as that of receiver 116 of user B stated above, the output of the auto-correlation characteristics shown in FIG. 11 are produced from matched filter 215. As a result, receiver 216 of user C recognizes that the received signal is addressed to the user C's station.
Meanwhile, the signal spread by the PN code Mc transmitted from transmitter 205 of user D is also inputted to receiver 116 of user B through transmission path 110. Consequently, the output of the cross-correlation characteristics between the inherent code Mc of user C and the inherent code Mb of user B are produced from matched filter 115.
FIG. 12 shows the concept of the cross-correlation characteristics of PN codes. The detail thereof is not described herein since it is described in, for example, “Spectrum Spread Communication System”, pp. 406-409, Mitsuo YOKOYAMA, Kagaku-Gijutsu Publishing company, INC., 1988. In short, the cross-correlation characteristics between different PN codes have various values according to the combinations of PN sequences and do not have fixed values as indicated by the auto-correlation characteristics shown in FIG. 11.
Therefore, matched filter 115 produces not only the output of the auto-correlation characteristics for detecting a signal addressed to the user B's station shown in FIG. 11, but also unnecessary output of the cross-correlation characteristics shown in FIG. 12. Generally, the cross-correlation characteristics among inherent codes such as Mb and Mc assigned to respective users are designed to take levels sufficiently lower than those of the auto-correlation characteristics, by making the codes not similar to one another.
Nonetheless, the conventional CDMA system using PN codes as spreading codes stated above has the following major problems. Since each user freely moves in mobile communication, there are some cases where the signal (interference wave) level (cross-correlation characteristics shown in FIG. 12) inputted to a user's receiver but addressed to a different station is higher than that (the auto-correlation characteristics shown in FIG. 11) addressed to the user's station, depending on the user's position. This is a well-known problem called ‘near-far problem’. If the problem occurs, the signal addressed to the user's station is masked by the interference wave and cannot be detected. Furthermore, communication disturbance occurs, such as caused by multi-path signals due to reflection waves which disturbs receiver detecting operation similarly to the interference waves.
To avoid the near-far problem, it is essential to appropriately control the transmission power levels of the respective transmitters in the overall system in accordance with the movement of the users. It has been disadvantageous that the power control makes the system constitution complicate and large in size.
The present invention has been made to solve the above-stated problems on the conventional CDMA communication systems. The invention is to make the matched filter output level (the cross-correlation characteristics between the interference wave and a desired station signal) zero when an interference wave is given to the input to thereby solve the near-far problem. It is, therefore, an object of the present invention is to provide a CDMA communication system which is composed of a simple constitution because it does not require the transmission power control of the respective transmitters, and is easily equipped with multi-path signal separation function.